Conventionally, with respect to a technique for coating the surface of a treatment target so as to provide corrosion resistant property and abrasion resistant property, for example, Japanese Patent Application Laid-Open (JP-A) No. 5-148615 has disclosed a discharging surface treatment method. In this technique, a primary treatment (deposition treatment) is carried out by using a green compact electrode made from WC powder and Co powder, etc., and after exchanging this to an electrode that is comparatively less susceptible to electrode consumption, such as a copper electrode, a secondary treatment (re-fusing treatment) is then carried out. Thus, this surface treatment method for a metal material is constituted by the two processes. This conventional technique provides a superior method for forming a hard coat film having a thickness of approximately several tens of μm on a steel plate. However, a problem with this technique is that it is difficult to form a hard coat film having high adhering strength onto a sintered material such as an ultra-hard alloy.
Next, referring to FIG. 7, an explanation will be given of a discharging surface treatment method for forming a hard coat film having high adhering strength even to an ultra-hard alloy that is disclosed by Japanese Patent Application Laid-Open (JP-A) No. 9-192937. Reference number 1 represents a green compact electrode formed by compressing and molding TiH2, and reference number 2 represents a treatment target. Reference number 3 represents a treatment vessel, and reference number 4 represents a treatment solution. Reference number 5 represents a switching element for switching voltage and current to be applied to the green compact electrode 1 and the treatment target 2. Reference number 6 represents a control circuit for controlling ON-OFF of the switching element 5, and reference number 7 represents a power supply. Reference number 8 represents a resistor, and reference number 9 represents a hard coat film that has been formed. By using a discharging surface treatment having such an arrangement, it is possible to form a hard coat film having a thickness of approximately several μm to several tens of μm on the surface of steel or ultra-hard alloy.
Moreover, Japanese Patent Application Laid-Open (JP-A) No. 10-225824 has disclosed a method in which: a material that generates a highly hard carbide, such as Ti, V, Nb and Ta, is used as an electrode to generate a discharge so that the surface of the treatment target is decarbonated to have a slightly rough surface (pre-treatment), and a discharge is generated by using a TiH2-type green compact electrode so as to carry out a surface treatment (main treatment) on the treatment target. This pre-treatment is carried out so as to provide easy adhesion of the coating material in the main treatment. Moreover, for the same purpose as this, another method has been disclosed in which: a pre-treatment is carried out under a condition where the TiH2-type green compact electrode has a negative polarity with a comparatively small discharging energy, and the same TiH2-type green compact electrode with higher discharging energy is then used so as to carry out the main treatment.
Any of the above-mentioned conventional techniques features that a green compact electrode is used. However, for the following three reasons, it is difficult to put these into practical use.
First, it is difficult to mold an electrode having a size suitable for practical use. In other words, in order to mold the electrode so as to have a size suitable for practical use in the surface treatment for a metal mold, etc., the capability of the pressing device has to be extremely increased, and since upon compressing and molding powder material, a pressure is not uniformly transmitted to the inside of the material to cause unevenness in the density, resulting in defects such as cracks. Moreover, the green compact electrode thus formed is susceptible to collapse in shape, making it difficult to apply this to the secondary treatment, and variations occur in the hard coat film to be formed on the treatment target, resulting in degradation in the quality.
Second, difficulty in dealing with the electrode material. In other words, powder of Ti and TiH2 is susceptible to oxidation, and in particular, TiH2 tends to change with time, that is, to cause hydrolysis even in the air, resulting in difficulty in dealing therewith. Moreover, when put into water, it generates hydrogen gas violently, raising a problem in dealing with waste electrodes.
Third, difficulty in providing a thick film. In other words, the conventional methods are only allowed to form a thickness in the range of several μm to several tens μm, and fail to form a hard coat film having a thickness exceeding this thickness that is required from the industrial point of view.
The third reason is explained in more detail below. The formation of a thin-film has been widely carried out by physical vapor deposition and chemical vapor deposition, etc., which are dry processes. However, the formation of a thick-film is hardly carried out by these methods, and at present, it has to be carried out by using a flame coating method, etc. The flame coating method builds up various materials on a treatment target, but the structure is coarse, with the result that it is not suitable for the application such as a coat film on a metal mold that requires precision and durability, and it also has many limitations to materials to be applied.